Programmable logic integrated circuit architecture incorporating a global shareable expander

ABSTRACT

A logic element for a programmable logic device to implement a global shareable expander. The logic element includes logic modules (P0-P4) for implementing combinatorial logic and a register (445). The combinatorial and registered paths of a logic element may be utilized at the same time. The logic modules may be programmably coupled to the register. The output of the register may be programmably coupled through an output buffer (515) to an I/O pad (520) of the integrated circuit. The logic modules may bypass the register and directly programmably couple through the output buffer to the I/O pad. A logic module may be used as a shareable expander by programmably coupling the module through to a global interconnect with other logic modules in LABs coupled to the global interconnect.

This application claims the benefit of a provisional application Ser.No. 60/011,422, filed Apr. 9, 1996, and a provisional application Ser.No. 60/014,629, filed Apr. 9, 1996, both incorporated herein byreference.

BACKGROUND OF THE INVENTION

The present invention relates to the field of integrated circuits andtheir operation. More specifically, in one embodiment the inventionprovides an improved logic device, as well as an improved method ofoperating a logic device.

Logic devices and methods of their operation are well known to those ofskill in the art. Programmable logic devices have found particularlywide application as a result of their combined low up-front cost andversatility to the user.

Altera's FLEX® and MAX® lines of programmable logic are among the mostadvanced and successful programmable logic devices. In the FLEX® 8000logic devices, for example, a large matrix of logic elements (LEs) isutilized. In one commercial embodiment of such devices, each LE includesa 4-input look-up table for performance of combinational logic (e.g;AND, OR, NOT, XOR, NAND, NOR, and many others) and a register thatprovides sequential logic features.

The LEs are arranged in groups of, for example, eight to form largerlogic array blocks (LABs). The LABs contain, among other things, acommon interconnection structure. The various LABs are arranged in atwo-dimensional array, with the various LABs connectable to each otherand to pins of the device though continuous lines that run the entirelength/width of the device. These lines are referred to as rowinterconnect (GH) and column interconnect (GV) or "global" interconnectlines. In Altera's line of products these may include what are referredto as "Horizontal Fast Tracks™", and "Vertical Fast Tracks™."

The MAX® 7000 logic devices by way of contrast utilize what are commonlyreferred to as "macrocells" (analogous to LEs) as a basic logic element.The macrocells are arranged in groups of, for example, sixteen to formlarger logic array blocks (LABs). A programmable interconnect array(PIA) selectively links together the multiple LABs. The PIA is a globalbus that is fed by all dedicated inputs, I/O pins, and the variousmacrocells. The PIA is analogous to global interconnect, GHs and GVs.For example, the PIA may be fed by signals that will be used as logicinputs, global controls for secondary register functions in the LABs,input paths from I/O pins to registers that are used for setup of thedevice, etc.

Inputs to the LABs include inputs from pins (via I/O control blocks),the PIA, and various control (e.g. clock) pins. Logic inputs areprovided to one or more of five AND devices, the outputs of which areprovided to a product term select matrix. The product term select matrixselects which inputs will be provided to an OR or XOR function, or assecondary inputs to registers in the macrocell. Product terms may beshared between macrocells for complex logic functions. Outputs from theLABs are provided to the I/O control block to the PIA and/or variousoutput pins.

The FLEX® and MAX® logic devices have met with substantial success andare considered pioneering in the area of programmable logic. Whilepioneering in the industry, certain limitations still remain. Forexample, the present invention recognizes that it would be desirable tofurther increase the flexibility of such devices to perform logic.

From the above it is seen that an improved programmable logic device andmethod of operation therefore is desired.

SUMMARY OF THE INVENTION

The present invention is a logic element or macrocell for a programmablelogic device which provides a shareable expander feature where Logicmodules and product terms in a particular LAB may be combined using aglobal interconnect with logic modules and product terms in other LABswithin the programmable logic device. The logic element may alsoincorporate a lonely register feature so that combinatorial andregistered functions may be implemented more efficiently in a singlelogic element. These features improve the overall utilization of theresources of the programmable logic device. Consequently, logic may bemore densely packed in the PLD, and an individual PLD may be used toimplement more complex functions and operations.

In particular, the logic element includes logic modules for implementingcombinatorial logic and a register. The combinatorial and registeredpaths of a logic element may be utilized at the same time. The logicmodules may be programmably coupled to the register. The output of theregister may be programmably coupled through a programmable interconnectarray to an output buffer and an I/O pad of the integrated circuit. Thelogic modules may bypass the register and directly programmably couplethrough the programmable interconnect array. A logic module mayprogrammably couple through the programmable interconnect array to oneor more output buffers. Some logic elements may directly couple to anoutput buffer and I/O pad.

A logic module may also be used as a shareable expander by programmablycoupling the module through to a global interconnect and combining itwith other logic modules in LABs coupled to the global interconnect. Byusing a logic module as a shareable expander, the logic module may stillbe used for other functionality at the same time. In particular, thelogic module may have a regular logic module output and a shareableexpander output.

In accordance with the teachings of this invention, a logic element fora programmable logic device is disclosed. The logic element includes afirst multiplexer which is coupled to the programmable interconnectarray of the programmable logic device. A plurality of logic modules orproduct terms is coupled to the first multiplexer. The logic modules maybe programmably configured to implement combinatorial logic functions. Aregister is coupled to the first multiplexer and the plurality of logicmodules. The register allows the implementation of registered functions.A particular logic module may be programmably coupled to the registerwhile other logic modules are programmably coupled to the firstmultiplexer, bypassing the register.

Other objects, features, and advantages of the present invention willbecome apparent upon consideration of the following detailed descriptionand the accompanying drawings, in which like reference designationsrepresent like features throughout the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a system incorporating a programmable logicdevice;

FIG. 2 is a block diagram showing the overall architecture of aprogrammable logic device incorporating the present invention;

FIG. 3 is a simplified block diagram of a logic array block (LAB) of aprogrammable logic device; and

FIG. 4 is a diagram of a logic element of the present inventionincorporating a global shareable expander and a lonely register.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a block diagram of a digital system within which thepresent invention may be embodied. The system may be provided on asingle board, on multiple boards, or even within multiple enclosures.FIG. 1 illustrates a system 101 in which a programmable logic device 121may be utilized. Programmable logic devices (sometimes referred to as aPALs, PLAs, FPLAs, PLDs, EPLDs, CPLDS, EEPLDs, LCAs, or FPGAs), are wellknow integrated circuits that provide the advantages of fixed integratedcircuits with the flexibility of custom integrated circuits. Suchdevices allow a user to electrically program standard, off-the-shelflogic elements to meet a user's specific needs. See, for example, U.S.Pat. No. 4,617,479, incorporated herein by reference for all purposes.Such devices are currently represented by, for example, Altera's MAX®series of PLDs and FLEX® series of PLDs. The former are described in,for example, U.S. Pat. No. 5,241,224 and 4,871,930, and the Altera DataBook, June 1996, all incorporated herein by reference. The latter aredescribed in, for example, U.S. Pat. No. 5,258,668, 5,260,610, 5,260,611and 5,436,575, and the Altera Data Book, June 1996, all incorporatedherein by reference for all purposes. Logic devices and their operationare well known to those of skill in the art.

In the particular embodiment of FIG. 1, a processing unit 101 is coupledto a memory 105 and an I/O 111 and incorporates a programmable logicdevice (PLD) 121. PLD 121 may be specially coupled to memory 105 throughconnection 131 and to I/O 111 through connection 135. The system may bea programmed digital computer system, digital signal processing system,specialized digital switching network, or other processing system.Moreover, such systems may be designed for a wide variety ofapplications such as, merely by way of example, telecommunicationssystems, automotive systems, control systems, consumer electronics,personal computers, and others.

Processing unit 101 may direct data to an appropriate system componentfor processing or storage, execute a program stored in memory 105 orinput using I/O 111, or other similar function. Processing unit 101 maybe a central processing unit (CPU), microprocessor, floating pointcoprocessor, graphics coprocessor, hardware controller, microcontroller,programmable logic device programmed for use as a controller, or otherprocessing unit. Furthermore, in many embodiments, there is often noneed for a CPU. For example, instead of a CPU, one or more PLDs 121 maycontrol the logical operations of the system. In some embodiments,processing unit 101 may even be a computer system. Memory 105 may be arandom access memory (RAM), read only memory (ROM), fixed or flexibledisk media, PC Card flash disk memory, tape, or any other storageretrieval means, or any combination of these storage retrieval means.PLD 121 may serve many different purposes within the system in FIG. 1.PLD 121 may be a logical building block of processing unit 101,supporting its internal and external operations. PLD 121 is programmedto implement the logical functions necessary to carry on its particularrole in system operation.

FIG. 2 is a simplified block diagram of the overall internalarchitecture and organization of PLD 121 of FIG. 1. Many details of PLDarchitecture, organization, and circuit design are not necessary for anunderstanding of the present invention and such details are not shown inFIG. 2.

FIG. 2 shows a six-by-six two-dimensional array of thirty-six logicarray blocks (LABs) 200. LAB 200 is a physically grouped set of logicalresources that is configured or programmed to perform logical functions.The internal architecture of a LAB will be described in more detailbelow in connection with FIG. 3. PLDs may contain any arbitrary numberof LABS, more or less than the PLD 121 shown in FIG. 2. Some PLDs mayeven contain a single LAB. However, generally in the future, astechnology advances and improves, programmable logic devices with evengreater numbers of logic array blocks will undoubtedly be created.

Furthermore, LABs 200 need not be organized in a square or rectangularmatrix. While a rectangular or square array is generally an efficientlayout structure, any arrangement of LABs inside the PLD may beconceived. For example, the array may be organized in a five-by-seven ora twenty-by-seventy matrix of LABs. There are also many variations ofthis architecture. For example, a PLD may have multiple rows of LABs intwo columns. There may also be an arrangement of 1-by-N LABs in a PLD,where N is an integer. In a very basic architecture, there may only beGV lines or only GH lines since signals do not need to be interconnectedto a third column. Specifically, a PLD may have two columns by anynumber of rows of interconnect. As a further example, a PLD may have onecolumn and any number of rows of interconnect. Furthermore, in somecircumstances, some number of LABs may be replaced by different types ofprogrammable structures.

LAB 200 has inputs and outputs (not shown) which may or may not beprogrammably connected to a global interconnect structure, comprising anarray of global horizontal interconnects (GHs) 210 and global verticalinterconnects (GVs) 220. Although shown as single lines in FIG. 2, eachGH 210 and GV 220 line represents a plurality of signal conductors. Theinputs and outputs of LAB 200 are programmably connectable to anadjacent GH 210 and an adjacent GV 220. Utilizing GH 210 and GV 220interconnects, multiple LABs 200 may be connected and combined toimplement larger, more complex logic functions than can be realizedusing a single LAB 200. In some products, the global interconnect may bereferred to as a global interconnect array (GIA) or a programmableinterconnect array (PIA).

In one embodiment, GH 210 and GV 220 conductors may or may not beprogrammably connectable at intersections 225 of these conductors. Also,in some embodiments, intersection 225 may have programmable drivers forselecting the signal from a conductor in one direction and buffer thesignal and drive it onto one of the alternate conductors in the same ordifferent direction. Moreover, GH 210 and GV 220 conductors may makemultiple connections to other GH 210 and GV 220 conductors. Various GH210 and GV 220 conductors may be programmably connected together tocreate a signal path from a LAB 200 at one location on PLD 121 toanother LAB 200 at another location on PLD 121. Furthermore, an outputsignal from one LAB 200 can be directed into the inputs of one or moreLA.Bs 200. Also, using the global interconnect, signals from a LAB 200can be fed back into the same LAB 200. In other embodiments or thepresent invention, only selected GH 210 conductors are programmablyconnectable to a selection of GV 220 conductors. Furthermore, in stillfurther embodiments, GH 210 and GV 220 conductors may be specificallyused for passing signal in a specific direction, such as input oroutput, but not both. GH 210 and GV 220 conductors may also pass signalsin multiple directions. For example, a global conductor may be used as abidirectional tristate bus.

In particular embodiments, the global interconnect may contain long andsegmented conductors. Long conductors run the entire length or width ofPLD 121. In particular, long conductors may programmably couple LABsalong a length or width of PLD 121. Segmented conductors are for shorterlength interconnections. For example, segmented conductors may includedouble lines for interconnections between two LABs 200. Other segmentedconductors include, among other, triple lines, quadruple lines,quintuple lines, sextuple lines, and other similar interconnectionresources. Furthermore, at intersections 225, segmented conductors maybe programmably coupled (or programmably uncoupled) to other long orsegmented conductors, in the same or different direction. Intersection225 may sometimes be referred to as a "switch box." As an example, adouble line may be programmably coupled to other double, long, orsegmented lines, in the same or different direction, at intersections225.

The PLD architecture in FIG. 2 further shows at the peripheries of thechip, input-output drivers 230. Input-output drivers 230 are forinterfacing the PLD to external, off-chip circuitry. FIG. 2 showsthirty-two input-output drivers 230; however, a PLD may contain anynumber of input-output drivers, more or less than the number depicted.Each input-output driver 230 is configurable for use as an input driver,output driver, or bidirectional driver.

FIG. 3 shows a simplified block diagram of LAB 200 of FIG. 2. LAB 200 iscomprised of a varying number of logic elements (LEs) 300, sometimesreferred to as "logic cells" or "macrocells." Some embodiments of LAB200 may include a local (or internal) interconnect structure 310. Thelocal interconnect is analogous to a local PIA. Some embodiments of LABs201 do not include a local interconnect structure, but may use GVs, GHs,and PIA, to name just a few. LAB 200 has eight LEs 300, but LAB 200 mayhave any number of LEs, more or less than eight. In a further embodimentof the present invention, LAB 200 has two "banks" of eight LEs for atotal of sixteen LEs, where each bank has common inputs, but separateoutputs and control signals. In some embodiments, LAB 200 includes carrychains or parallel expander chains.

A general overview of LE 300 is presented here, sufficient to provide abasic understanding of LAB 200. LE 300 is the smallest logical buildingblock of a PLD. Signals external to the LAB, such as from GHs 210 andGVs 220, are programmably connected to LE 300 through local interconnectstructure 310, although LE 300 may be implemented in many architecturesother than those shown in FIGS. 1-3. In one embodiment, LE 300 of thepresent invention incorporates a function generator that is configurableto provide a logical function of a number of variables, such afour-variable Boolean operation. As well as combinatorial functions, LE300 also provides support for sequential and registered functions using,for example, D flip-flops. The function generator may be implementedusing product terms, look-up tables, logic gates, logic modules, andothers.

LE 300 provides combinatorial and registered outputs that areconnectable to the GHs 210 and GVs 220, outside LAB 200. Furthermore,the outputs from LE 300 may be internally fed back into localinterconnect structure 310; through local interconnect structure 310, anoutput from one LE 300 may be programmably connected to the inputs ofother LEs 300, without using the global interconnect structure's GHs 210and GVs 220. Local interconnect structure 310 allows short-distanceinterconnection of LEs, without utilizing the limited global resources,GHs 210 and GVs 220. Through local interconnect structure 310 and localfeedback, LEs 300 are programmably connectable to form larger, morecomplex logical functions than can be realized using a single LE 300.Furthermore, because of its reduced size and shorter length, localinterconnect structure 310 has reduced parasitics compared to the globalinterconnection structure. Consequently, local interconnect structure310 generally allows signals to propagate faster than through the globalinterconnect structure.

FIG. 4 is a diagram of a logic element or macrocell of the presentinvention. The logic element includes a plurality of AND gates 410, 412,414, 416, and 418. Each of the plurality of AND gates is associated witha respective plurality of inputs P0, P1, P2, P3, and P4, which may bereferred to as product terms. Product terms and AND gates implement atype of logic module for performing logical functions. In this case,product terms P0-P4 provide programmable AND functionality. Depending onwhat inputs are programmable coupled to the products terms, differentlogic functions may be implemented. In further embodiments of thepresent invention, other types of logic modules may be used. These logicmodules would provide other functionality including look-up tables,RAMs, FIFOs, registers, logic gates, and many others.

In the present invention, the product terms inputs may be programmablycoupled to signals on the local interconnect of the LAB as well as theglobal interconnect and signals external to the LAB. For example,product term P0 may be coupled to a plurality of signals such as A1, A2,A3, . . . Ai as well as complements of these signals. An output of AND410 would be an AND function of the inputs programmably coupled to ANDgate 410.

In a specific embodiment, product term P0, which is a shareable productterm expander, may be coupled to local interconnect 310 and not theglobal interconnect. Many other interconnect variations are possible.

The outputs of the AND gates are further coupled to a plurality of3-to-2 multiplexers 420a-e. These multiplexers are used to programmablycouple the product terms to other components in the LE. Each multiplexeris programmably controlled by way of a respective programmable bit(i.e., SB0, SE1, SE2, SB3, SB4). These programmable bits may beimplemented using cells from RAM, DRAM, SRAM, EEPROM, EPROM, fuse,antifuse, ferro-magnetic, and other technologies. For example, aprogrammable bit may comprise a plurality of SPAM or EEPROM cells. Afirst output of each multiplexer is coupled to an OR gate 423. For thisfirst output, each multiplexer selects from one of two inputs, VSS or anoutput from a respective AND gate product term, to couple to OR gate423. For example, multiplexer 420c may programmable couple the outputfrom AND gate 414 (product term P2) to an input of OR gate 423. Usingmultiplexers 420a-e, inputs to OR gate 423 may also be programmablycoupled on VSS, which may be used to disable an unused input of OR gate423. An input PTAI to OR gate 423 is a logical output from a differentLE. In a specific embodiment, PTAI is an output from an OR gate in aneighboring LE. This path allows interconnections of the logic elementswithout using the local or global interconnect.

As those of skill in the art will recognize, by using the programmableAND gates and OR gate of the present invention, many different logicfunctions may be programmably implemented. In other embodiments of theinvention, however, logic functions may also be programmably performedusing other types of components and logic modules including NAND gates,OR gates, XOR gates, look-up tables, Flip-flops, registers, FIFOs, RAMs,and SRAMs, to name a few.

A second output from multiplexer 420a, a second output from multiplexer420b, and an output from OR gate 423 is coupled to a multiplexer 425.Multiplexer 425 is controlled by way of a programmable bit SB7. Theoutput of OR gate 423 is also coupled to a multiplexer 427, which isalso controlled by programmable bit SB7. Multiplexer 427 couples VSS orthe output of OR gate 423 to PTA0, which passes logical information toother logic elements, without needing to use the local or globalinterconnect. In a specific embodiment, PTA0 couples to only one otherlogic element.

Depending on a state of programmable bit S20, the second output frommultiplexer 420a programmable couples VDD or the output of AND gate 410to multiplexer 425. Similarly, depending on a state of programmable bitSB1, multiplexer 420b couples VDD or the output of AND gate 412 tomultiplexer 425. For example, product term P0 may be programmablecoupled through multiplexer 420a to multiplexer 425. A first output frommultiplexer 425 is used to programmably couple OR gate 423 ormultiplexer 420b to an XOR gate 430. A second output from multiplexer425 is used to programmably couple multiplexer 420a or multiplexer 420bto a 4-to-1 multiplexer 433. An output of 4-to-1 multiplexer 433 iscoupled to an input of XOR gate 430. An output from XOR gate 430 iscoupled to multiplexer 436. XOR gate 430 performs an XOR of the outputof OR gate 423 and the output of multiplexer 433. Among other functionaland logical uses, XOR gate 430 may be used to invert the polarity oflogic. For example, XOR gate 430 may invert the output of OR gate 423before passing the output signal to other components in the logicelement. Multiplexer 436 is also coupled to the second output ofmultiplexer 420a. Multiplexer 436 is controlled by way of an NOR 438 ofprogrammable bits SB10 and SB13.

The second output of multiplexer 420a may also be programmably coupledthrough a multiplexer 470 and an inverter 473 to generate an EXPWLsignal. This is a shareable expander feature. Shareable expanders may beused to implement NAND and logic, which provides further logicalcapabilities when ANDed into the product terms for another logic arrayblock. The EXPWL signal feeds back to the local interconnect of the LABto enable the creation of expanded logical functions. For example,product term P0 may be further combined with other product terms ofother LEs in the LAB. In such a fashion, shareable expanders allowsproduct terms to be expanded by sharing them among the LEs in a LAB.This feature allows logic cells to use uncommitted product terms andshare them among other product terms in the LAB. This improves theoverall efficiency in utilizing the resources of the programmable logicdevice. The EXPWL signal may also be coupled through another LE to theglobal interconnect to allow expanded product terms across differentLABs. Controlled by way of a programmable bit SB0, multiplexer 470 mayalso be used to provide a VDD for the EXPWL signal (i.e., VSS passesthrough multiplexer 470 and inverter 473) when the shareable expanderfeature is not used. Inverter 473 provides some buffering between thecomponents.

Furthermore, in this embodiment of the present invention, the shareableexpander feature may be used with other LABs in a PLD without passingthrough the local interconnect. More specifically, for example, productterm P0 may be used as a product term expander by programmably couplingit through multiplexer 420a and a 3-to-1 multiplexer 440, which isprogrammably controlled by programmable bit SB10, to the globalinterconnect. Using the global interconnect, product term P0 may beprogrammably coupled with product terms in other LABs. An advantage ofthis feature is that a LAB can lend some of its shareable expanders toother LABs. A LE can "donate" a shareable expander onto the globalinterconnect, giving more freedom to spread out the shareable expanderusage among LABs. This feature allows the creation of a multitude oflarger, more complex logical functions without wasting interconnectionand logical resources.

Furthermore, when multiplier 440 is used to place a shareable expanderon the global interconnect, multiplexer 458 will still be available sothat the logic element can be used to provide combinatorial orregistered logic. This allows further flexibility in the implementationof logical functions, and increases the overall available functionality.

The output from XOR gate 430 is further coupled to 3-to-1 multiplexer440. An output from multiplexer 440 may be coupled to the globalinterconnect (i.e., GHs and GVs). In further embodiments of the presentinvention, multiplexer 440 may also be coupled to the local interconnectof a LAB. For example, XOR gate 430 may be programmably coupled throughmultiplexer 440 and a GH line to provide logical signals for other LABsand LEs.

Multiplexer 436 may be programmably coupled through a multiplexer 442 toan input of a register 445. In a preferred embodiment, register 445 is aD-type register and multiplexer 442 is coupled to a D input of register445. The D input is used for inputting data. However, in otherembodiments of the present invention, other types of storage blocks maybe used for register 445 including various types of latches, flip-flops,registers, memory cells, and many others. For example, register 445 maybe a T flip-flop, S-R flip-flop, J-K flips-flop, or transparent latch.

A Q output of register 445 may be fed back through multiplexer 442 tothe D input. The Q output may feed back to XOR gate 430 through 4-to-1multiplexer 433, programmably controlled by programmable bits SB8 andSB9. The Q output may be coupled through multiplexer 440 to the globalinterconnect. A QB output of register 445 may programmably feedbackthrough 4-to-1 multiplexer 433 to XOR gate 430. The QB output is thecomplement of the Q output.

The Q output may be fed back to the local interconnect through amultiplexer 458, programmably controlled by programmable bit SB13.Further, through multiplexer 458, the output of XOR gate 430 may be fedback to the local interconnect of the LAB. This path from XOR gate 430bypasses register 445. Multiplexer 458 couples to the local interconnectof a LAB without needing to utilize the global interconnect. The pathalso includes a buffer 460 to provide the true and complement of theparticular signal to the local interconnect. This allows greaterflexibility when implementing logical functions.

A CLK input of register 445 is used for clocking. A first output of amultiplexer 450, controlled by way of programmable bit SB5, is used tocouple a second output of multiplexer 420d or an output of multiplexer454 to the CLK input. Through multiplexer 420d, product term P3 or VDDmay be used to programmably control the CLK input. Alternatively, globalclock signals GCLK0 or GCLK1 may be used to control the CLK inputthrough multiplexer 454, programmably controlled by a programmable bitSB12. In a preferred embodiment, the global clock signals may beconnected to a plurality of LABs and LEs on the chip through direct,dedicated lines without need to couple through the global and localinterconnect.

A second output of multiplexer 450 is used to control the selection ofmultiplexer 442, which determines the D input of register 445. Productterm P3 or VDD may be used to control the selection of multiplexer 442.

A CLR input is used to clear register 445. In a preferred embodiment,CLR is active low. A multiplexer 463, controlled by a programmable bitSB6, couples a second output of multiplexer 420c or an output of OR gate465 through an inverter 464 to the CLR input. Through multiplexer 420c,product term P2 or VSS may be coupled to control the CLR input.Alternatively, via OR gate 465, the second output of multiplexer 420cmay be ORed with an inverse of an NGCLR signal to control the CLR input.The NGCLR signal may be used as a global signal to implement a globalclear signal to clear a plurality of registers 445 for a plurality ofLEs.

A PRE input is used to preset register 445. In a preferred embodiment,the PRE input is active low. The PRE input is controlled using a secondoutput from multiplexer 420e. Through multiplexer 420e, product term P4or VSS may be programmably coupled to control the PRE input. Moreover,the CLR input may "override" the PRE input. To implement this, in aspecific embodiment, the PRE input is coupled to an output of a NANDgate 468. NAND gate 468 has an input from an output of inverter 464 andan input from a second output of multiplexer 420e. Other embodiments mayuse different logical and circuit configurations to implement thesefunctions.

The implementation shown in FIG. 4 merely illustrates the principles ofthe present invention, and many various circuit embodiments arepossible. For example, local shareable expander path may be implementedby changing multiplexer 458 into a 3-to-1 multiplexer. Specifically, asin the case of multiplexer 440, multiplexer 453 would be coupled to thesecond output of multiplexer 420a. Then, product term P0 may utilizethis feedback path through multiplexer 458 as a local shareable expanderpath back to the local interconnect. The dedicated shareable expanderpath, through multiplexer 470 and inverter 473, may be removed. However,the embodiment shown in FIG. 4 is generally preferred because it allowsthe feedback path through multiplexer 458 to be used at the same timethe dedicated shareable expander path is used.

Also, multiplexer 458 may connect to the local or global interconnectsby expanding this interconnect resource to include the output ofmultiplexer 458; this does not require a new dedicated line. Further,the output of multiplexer 458 may be coupled to this multiplexer in muchthe same way as other lines in the local or global interconnect.

The present invention also implements a lonely register architecture.Some LEs or macrocells may be used for both registered and combinatoriallogic by appropriately configuring multiplexers 440 and 458.

The foregoing description of preferred embodiments of the invention hasbeen presented for the purposes of illustration and description. It isnot intended to be exhaustive or to limit the invention to the preciseform described, and many modifications and variations are possible inlight of the teaching above. The embodiments were chosen and describedin order to best explain the principles of the invention and itspractical applications to thereby enable others skilled in the art tobest utilize the invention in various embodiments and with variousmodifications as are suited to the particular use contemplated. It isintended that the scope of the invention be defined by the claimsappended hereto.

What is claimed is:
 1. A programmable logic device comprising:aprogrammable interconnect array; and a plurality of logic array blocksprogrammably coupled to the programmable interconnect array, wherein alogic array block comprises:a plurality of product terms programmablyconfigured to perform combinatorial logic; a register programmablycoupled to the plurality of product terms; and a first multiplexercomprising, a first input coupled to an output of the register and asecond input directly coupled to a shareable product term of theplurality of product terms without passing through the register, whereinthe shareable product term may be coupled through the first multiplexerto the programmable interconnect array and programmably combined withother product terms.
 2. The programmable logic device of claim 1 whereinthe first multiplexer may programmably couple the register to theprogrammable interconnect array.
 3. The programmable logic device ofclaim 1 wherein the logic array block further comprises:an OR gateprogrammably coupled to the plurality of product terms, wherein an ORgate output may be programmably coupled through the first multiplexer toa local interconnect of the logic array block, and the shareable productterm is directly coupled to the second input of the first multiplexerwithout passing through the OR gate.
 4. The programmable logic device ofclaim 2 wherein the logic array block further comprises:a secondmultiplexer coupled to the output of the register and the OR gateoutput, wherein the OR gate output may be programmably coupled throughthe second multiplexer to a local interconnect of the logic array block.5. A digital system incorporating a programmable logic device as recitedin claim
 1. 6. The programmable logic device of claim 1 wherein eachlogic array block further comprises an inverter between the shareableproduct term and the second input of the first multiplexer, wherein theinverts a signal from the shareholder product term.
 7. A programmablelogic device having a plurality of logic array blocks programmablycoupled by a programmable interconnect array, each logic array blockhaving a plurality of macrocells, each macrocell comprising:a pluralityof logic modules programmably configured to perform logic; an OR gatecoupled to the plurality of logic modules; a register coupled to the ORgate; a first multiplexer comprising a first input coupled to the ORgate, a second input coupled to the register, and a third input coupledto a shareable logic module of the plurality of logic modules, whereinthe first multiplexer programmably couples the shareable logic module toa programmable interconnect array to be programmably combined with alogic module in one of the plurality of logic array blocks.
 8. Theprogrammable logic device of claim 7 wherein each macrocell furthercomprises:a second multiplexer coupled to the OR gate and the register,wherein the second multiplexer programmably couples the shareable logicmodule, register, or an XOR gate to the programmable interconnect array.9. The programmable logic device of claim 7 wherein each macrocellfurther comprises:a second multiplexer coupled to the OR gate and theregister, wherein the second multiplexer programmably couples theshareable logic module of the plurality of logic modules to the logicmodules in a logic array block without using the programmableinterconnect array.
 10. The programmable logic device of claim 7 whereinthe OR gate may be programmably coupled through the first multiplexer tothe programmable interconnect array, bypassing the register.
 11. Theprogrammable logic device of claim 7 further comprising:an XOR gate,programmably coupled between the OR gate or one of the plurality oflogic modules and the register.
 12. A programmable logic devicecomprising:a programmable interconnect array; and a plurality of logicarray blocks programmably coupled to the programmable interconnectarray, wherein a logic array block comprises:an output multiplexer; alogic gate; and a plurality of product terms programmably configurableto perform combinatorial logic, coupled to the logic gate, wherein oneof the plurality of product terms may be directly coupled to an input ofthe output multiplexer without going through the logic gate to theprogrammable interconnect array and programmably combined with otherproduct terms.
 13. The programmable logic device of claim 12 wherein thelogic array block further comprises:a register, coupled to the pluralityof product terms, configurable to perform registered logic, wherein theplurality of product terms may bypass the register to programmablycouple to the programmable interconnect array.
 14. The programmablelogic device of claim 12 wherein each logic array block furthercomprises an inverter coupled between the one of the plurality ofproduct terms and the input of the output multiplexer.
 15. Theprogrammable logic device of claim 12 wherein a logic array blockfurther comprises:a logic gate, wherein the plurality of product termsare coupled to inputs of the logic gate, and the one of plurality ofproducts is directly coupled to the input of the output multiplexerwithout passing through the logic gate.
 16. The programmable logicdevice of claim 15 wherein the logic gate implements an OR function andthe product terms implement an AND function.
 17. A programmableintegrated circuit comprising:a plurality of logic modules configurableto perform logic functions, wherein one of the plurality of logicmodules is a shareable logic module; a logic gate, coupled to theplurality of logic modules; a register, coupled to the logic gate; andan output multiplexing circuit comprising a first input coupled to theshareable logic module, a second input coupled to the logic gate, and athird input coupled to the register.
 18. The programmable integratedcircuit of claim 17, further comprising an inverter coupled between theshareable logic module and the first input of the output multiplexer.19. The programmable integrated circuit of claim 17 wherein the logicmodules perform an AND function, and the logic gate performs an ORfunction.
 20. The programmable integrated circuit of claim 17 furthercomprising a second output multiplexing circuit comprising a first inputcoupled to the logic gate and a second input coupled to the register.21. The programmable integrated circuit of claim 17 wherein the outputmultiplexing circuit selectively couples its first input, second input,or third input to a programmable interconnect line.